The present invention relates to a semiconductor device, and more particular, to a semiconductor memory apparatus with a delay locked loop circuit.
A semiconductor memory apparatus embodied in a system which includes a plurality of semiconductor devices is used for storing data. The semiconductor memory apparatus outputs data stored in a cell corresponding to an address outputted from a data requesting unit such as a central processing unit or stores data provided from the data requesting unit into the cell.
As an operational speed of a system including a plurality of semiconductor devices has increased and a technology related to a semiconductor integrated circuit has advanced, there has been a demand for increasing data access speed of the semiconductor memory apparatus. In order to access data of semiconductor memory apparatus with a high speed, a synchronous memory device which receives a system clock and can access data every time synchronized with transition of the received system clock has been proposed. Nevertheless, the synchronous memory device cannot meet data access speed required by the system, particularly, the data requesting unit included in the system. Accordingly, a double data rate (DDR) synchronous semiconductor memory apparatus is proposed which can access data every rising edge and falling edge of the system clock.
The DDR synchronous semiconductor memory apparatus should receive or output two data within one cycle of the system clock, since the DDR synchronous semiconductor memory apparatus accesses one data every transition of the system clock. That is, the DDR synchronous semiconductor memory apparatus should output or receive data every time exactly synchronized with the rising edge and falling edge of the system clock. Typically, the output circuit of the DDR synchronous semiconductor memory apparatus receives the system clock through a clock transfer path including a clock buffer, clock transmission lines, and the like inside the DDR synchronous semiconductor memory apparatus and outputs data synchronized with the received system clock.
However, the system clock reaches the output circuit of the DDR synchronous semiconductor memory apparatus with an essential delay occurring while passing through the clock input buffer and the clock signal transmission lines arranged in the DDR synchronous semiconductor memory apparatus. Although the output circuit outputs data in synchronization with the reached system clock, data delayed by the essential delay are inputted to an external device receiving data from the DDR synchronous semiconductor memory apparatus.
To solve above described problem, the DDR synchronous semiconductor memory apparatus includes a circuit, i.e., a delay locked loop circuit for adjusting the essential delay by locking a delay of the transferred system clock. The delay locked loop compensates for a delay value caused by the clock transfer path of the semiconductor memory apparatus. The delay locked loop circuit detects a delay value which occurs while an inputted system clock passes through the clock transfer path including the clock input circuit, the clock signal transmission lines, and the like and delays the inputted system clock for a delay time corresponding to the detected delay value to thereby output the delayed system clock into the output circuit. That is, the delay locked loop circuit intentionally delays and locks the inputted system clock by a constant delay value depending on an amount of the detected delay value to output the delay locked clock into the output circuit. Then, the output circuit outputs a data synchronized with the delay locked clock. The data outputted from the output circuit is exactly transferred to the external device, synchronized with the system clock.
Substantially, the delay locked clock is inputted to the output circuit at one previous cycle before the data is outputted; and, synchronized with the delay locked clock, the output circuit outputs the data. Namely, for synchronizing the outputted data with the system clock, the data should be outputted faster than the system clock is delayed on the clock transfer path. Outside the semiconductor memory apparatus, the outputted data are exactly synchronized with rising and falling edges of the system clock, input to the semiconductor memory apparatus. Consequently, the delay locked loop is a circuit for determining how much the data is outputted faster than the timing of data output in order to compensate the delay value of the system clock inside the semiconductor memory apparatus.
FIG. 1 illustrates a timing diagram describing an operation of a delay locked loop for use in a conventional semiconductor memory apparatus.
As shown, the delay locked loop generates a delay locked clock DLL_OUT whose phase leads that of inputted internal clock CLKI. Herein, the internal clock CLKI is generated based on an external clock CLK0 from a clock buffer or an internal clock generator. The semiconductor memory apparatus outputs data D0, D1, and D2, synchronized with the delay locked clock DLL_OUT. If the data D0, D1, and D2 are synchronized with the delay locked clock DLL_OUT, the data can be also synchronized with the external clock CLK0.
Generally, the delay locked loop includes a delay line for delaying an internal clock, a delay line controller for controlling a delay amount of the delay line, a delay model for delaying an output of the delay line by an estimated amount, i.e., how long the internal clock is delayed by a clock path inside the semiconductor memory apparatus, and a phase comparator for comparing a phase of the internal clock with that of an output of the delay model. If the delay locked loop achieves a locking state, i.e., is able to generate a proper delay locked clock for outputting data, a locking detector senses the locking state to generate a locking sense signal. When the locking detector generates the locking sense signal, the semiconductor memory apparatus outputs data in synchronization with transitions, i.e., rising and falling edges, of the delay locked clock generated from the delay locked loop.
If the locking detector may recognize the locking state and generate the locking sense signal when the delay locked loop does not achieve the locking state, the semiconductor memory apparatus receives wrong information and outputs data in synchronization with an incomplete delay locked clock. In this case, outputted data may not be synchronized with the system clock. As a result, a system or an external device can not receive the outputted data from the semiconductor memory apparatus.